The main lead bushings of an electrical generator function both as a conductor for the generated electrical energy and as a seal for the cooling gas within the generator. The typical design of a main lead bushing includes a cylindrical hollow copper conductor with cylindrical flanges attached to it. An air side flange is attached to the end of the segment of the hollow conductor that extends outside of the generator. A gas side flange is attached to the end of the segment that extends inside the generator and connects to the lead box.
The seal between the hollow conductor and the flanges is of critical importance, both for mechanical and electrical reasons. The main lead bushings are subject to vibrational loads during operation, and gaps in the seal create stress risers that may result in flange detachment and generator failure. Also, electrical current flows primarily along the outer surface of the conductor and outward along the surface of the flange. Accordingly, gaps or holes in the seal, especially near the outer surface of the conductor, may lead to flux concentrations and locally intensified heating, which also may result in generator failure.
FIG. 1 depicts a conductor 1 attached to a flange 2 in accordance with the prior art. In order to attach the flange 2 to the hollow conductor 1, a circular gap 3 sized to receive the end 4 of the conductor 1 is formed in the base of the flange. Threads 5, 6 are formed in both the outside surface of the end 4 of the conductor and in the mating surface of the gap 3 in the flange. The threads allow the conductor and the flange to be threaded together when the conductor is inserted into the gap. In order to facilitate ease of threading, a relatively large gap 3 is needed between the conductor and the flange 3. Also, the end 4 of the conductor is beveled 7 near the base to facilitate starting the threads, and a beveled slot 8 may be formed in the flange at the top of the gap on the outer side.
The conductor and the flange are cleaned using an acidic chemical substance such as Oakite-33 since the geometry of the threads makes mechanical cleaning processes ineffective. Flux is applied to the hollow conductor and the flange, and the end of the conductor is then inserted into the gap in the flange. The conductor and the flange are heated using either induction or furnace heating. Braze alloy is fed into the gap during heating. The braze alloy melts and flows within the gap so that, upon cooling, the flange and the conductor are bonded together.
However, several problems arise from using this method that prevent a complete bond from being formed. First, the threads in the connection create a tortuous path, so that any gas emanating from the heated flux is restricted from escaping. While some of the gas is forced up and out of the gap, much of it remains trapped in the gap. This makes it difficult for the braze alloy to flow into the bottom of the gap, as the flow of any gas is in the opposite direction from the flow of the braze material. Even if waterless flux is used, thereby reducing the volume of gas produced, the threads impede the flow of braze material to the bottom of the gap from the OD side of the flange. This is a major drawback as the seal between the conductor and the OD side is of critical importance since this is the primary flow path of current traveling through the conductor.
Further, application of braze material at the ID side of the flange is nearly impossible due to the long tubular shape of the conductor. Given the tortuous path through the thread and the difficulty of applying braze material to the ID side, it is virtually impossible that complete bonding can be accomplished along the bottom of the conductor. The bevel formed on the conductor OD to ease the starting of the threads aggravates this situation by creating a large void that should be filled with braze material.
The prior art method results in only about 15-20% of the surface of the flange and conductor being bonded. It is desirable to bond at least 75% of the surface to ensure structural integrity and electrical conductivity.
Accordingly, it is an object of this invention to provide a low cost, reliable process for constructing main lead bushings so as to form an improved seal between the flanges and the hollow conductor. This seal should be reliable and should provide a high degree of structural integrity and electrical conductivity between the flange and the hollow conductor.